There has been a lot of posting about what these two contexts are.. But I’m still not getting it quite right
As I understand it so far:
Each is an instance of its class which means that some programmers recommend you to use
this.getApplicationContext() as often as possible in order to not “leak” out any memory. This is because the other
this (getting the
Activity instance context) points to an
Activity that is being destroyed each time the user tilts the phone or leave the app etc.. Which apparently the Garbage Collector (GC) doesn’t catch and therefore uses too much memory..
But can anyone please come up with some really good coding examples where it would be the right thing to use
this (getting the context of the current
Activity instance) and the application context will be useless/wrong?
getApplicationContext() is almost always wrong. Ms. Hackborn (among others) have been very explicit that you only use
getApplicationContext() when you know why you are using
getApplicationContext() and only when you need to use
To be blunt, “some programmers” use
getBaseContext(), to a lesser extent) because their Java experience is limited. They implement an inner class (e.g., an
OnClickListener for a
Button in an
Activity) and need a
Context. Rather than using
MyActivity.this to get at the outer class’
this, they use
getBaseContext() to get a
You only use
getApplicationContext() when you know you need a
Context for something that may live longer than any other likely
Context you have at your disposal. Scenarios include:
getApplicationContext()if you need something tied to a
Contextthat itself will have global scope. I use
getApplicationContext(), for example, in
WakefulIntentService, for the static
WakeLockto be used for the service. Since that
WakeLockis static, and I need a
Contextto get at
PowerManagerto create it, it is safest to use
getApplicationContext()when you bind to a
Activity, if you wish to pass the
ServiceConnection(i.e., the handle to the binding) between
onRetainNonConfigurationInstance(). Android internally tracks bindings via these
ServiceConnectionsand holds references to the
Contextsthat create the bindings. If you bind from the
Activity, then the new
Activityinstance will have a reference to the
ServiceConnectionwhich has an implicit reference to the old
Activity, and the old
Activitycannot be garbage collected.
Some developers use custom subclasses of
Application for their own global data, which they retrieve via
getApplicationContext(). That’s certainly possible. I prefer static data members, if for no other reason than you can only have one custom
Application object. I built one app using a custom
Application object and found it to be painful. Ms. Hackborn also agrees with this position.
Here are reasons why not to use
getApplicationContext() wherever you go:
It’s not a complete
Context, supporting everything that
Activitydoes. Various things you will try to do with this
Contextwill fail, mostly related to the GUI.
It can create memory leaks, if the
getApplicationContext()holds onto something created by your calls on it that you don’t clean up. With an
Activity, if it holds onto something, once the
Activitygets garbage collected, everything else flushes out too. The
Applicationobject remains for the lifetime of your process.
I think there’s a lot of stuff that is poorly documented on the SDK site, this is one of them. The claim I’m going to make is that it seems as though it’s better to default to using an application context and only use an activity context when you really need to. The only place where I’ve ever seen that you need an activity context is for a progress dialog. SBERG412 claims that you have to use an activity context for a toast message, yet the Android docs clearly show an application context being used. I’ve always used application context for toasts because of this Google example. If it’s wrong to do so, then Google dropped the ball here.
Here’s more to think about and review:
For a toast message, the Google Dev Guide uses the application context and explicitly say’s to use it:
In the dialogs section of the Dev guide, you see that an AlertDialog.Builder uses the application context, and then the progress bar uses an activity context. This isn’t explained by Google.
It seems like a good reason to use application context is when you want to handle configuration changes like an orientation change, and you want to retain objects which need a context like Views. If you look here: Run Time Changes
There is a caution about using an activity context, which can create a leak. This can be avoided with an application context with the views that are to be retained (at least that’s my understanding). In an app I’m writing, I intend to use an application context because I’m trying to hold over some views and other things on an orientation change, and I still want the activity to be destroy and recreated on orientation changes. Thus I have to use an app context to not cause a memory leak (see Avoiding memory Leaks). To me it seems there are plenty of good reasons to use the application context instead of an activity context, and to me it almost seems like you would use it more often than an activity context. That’s what many Android books I’ve gone through seem to do, and that’s what much of the Google examples I’ve seen do.
The Google documentation really makes it seem like using application context is perfectly fine in most cases, and in fact appears more often than using an activity context in their examples (at least the examples I’ve seen). If it’s really such a problem to use application context, then Google really needs to place more emphasis on this. They need to make it clear, and they need to redo some of their examples. I wouldn’t blame this entirely on inexperienced developers since the authority (Google) really makes it look like it’s not a problem to use application contexts.
I used this table as a guidance for when to use the different types of Context such as Application context (i.e:
getApplicationContext()) and activity context , also BroadcastReceiver context:
All merits go to original author here for more info.
Which context to use?
There are two types of Context:
Application context is associated with the application and will always be same throughout the life of application — it does not change. So if you are using Toast, you can use application context or even activity context (both) because toast can be displayed from anywhere with in your application and is not attached to a specific window. But there are many exceptions, one exception is when you need to use or pass the activity context.
Activity context is associated with to the activity and can be destroyed if the activity is destroyed — there may be multiple activities (more than likely) with a single application. And sometimes you absolutely need the activity context handle. For example, should you launch a new activity, you need to use activity context in its Intent so that the new launching activity is connected to the current activity in terms of activity stack. However, you may use application’s context too to launch a new activity but then you need to set flag
Intent.FLAG_ACTIVITY_NEW_TASKin intent to treat it as a new task.
Let’s consider some cases:
MainActivity.thisrefers to the MainActivity context which extends Activity class but the base class (activity) also extends Context class, so it can be used to offer activity context.
getBaseContext()offers activity context.
getApplication()offers application context.
getApplicationContext()also offers application context.
For more information please check this link.
I was wondering why not to use Application Context for every operation which it supports. In the end it lowers chance of memory leak and missing null check for getContext() or getActivity() (when using injected application context or acquired through static method from Application). Statements, like the one by Ms. Hackborn to use Application Context only if needed, don’t seem convincing for me without an explanation why. But it seems that I have found an unswear why:
have found that there are issues on some Android version / device combinations that do not follow these rules. For instance, if I have a BroadcastReceiver that is passed a Context and I convert that Context to an Application Context and then try to call registerReceiver() on the Application Context there are many instances where this works fine, but also many instances where I get a crash because of a ReceiverCallNotAllowedException. These crashes occur on a wide range of Android versions from API 15 up to 22.
Two great examples of when you should use Activity context vs. the Application context are when displaying either a Toast message or a built-in Dialog message as using the Application context will cause an exception:
Toast t = Toast.makeText(this,....);
Both of these need information from the Activity context that is not provided in the Application context.
Application context live untill your application is alive only and it is not depend on Activity Life Cycle but, context keep object long-lived. If the object which you are used temporary, that time use Application Context and Activity Context is used totally oposite of Application Context.